Definition (D50)
Definition
Definition (D50)
Definition (D50) is an acoustic parameter that expresses the fraction of total impulse response energy arriving within the first 50 milliseconds as a percentage. Defined by ISO 3382-1:2009, D50 ranges from 0 to 100% and indicates how well a room supports speech intelligibility. Values above 50% indicate adequate definition. SonaVyx calculates D50 from measured impulse responses.
D50 = (∫₀⁵⁰ h²(t)dt / ∫₀∞ h²(t)dt) × 100%
How It Is Measured
D50 is computed from the impulse response by dividing the energy within the first 50 ms by the total impulse response energy. SonaVyx computes this automatically after impulse response capture. The metric is related to C50 by the formula: C50 = 10 × log₁₀(D50 / (1 - D50)). A D50 of 50% corresponds to C50 of 0 dB.
Practical Example
A classroom measures D50 of 38% at the back row, indicating poor speech definition. The teacher reports students struggling to understand lessons. After installing acoustic ceiling tiles and wall absorption panels, D50 improves to 62%. This corresponds to C50 improvement from -2.1 dB to +2.1 dB, bringing the room into the acceptable range for speech-focused spaces.
D50 vs C50
D50 and C50 contain the same information expressed differently. D50 is a linear ratio (percentage), while C50 is logarithmic (decibels). D50 is more intuitive for non-technical audiences — "65% of sound energy arrives within 50 ms" is easier to understand than "C50 = 2.7 dB." For technical analysis and comparison across rooms, C50 in dB is preferred because it better represents the perceptual scale.
Octave Band Analysis
D50 varies with frequency because absorption and reflection patterns are frequency-dependent. Low frequencies typically have lower D50 (longer RT60, more diffuse) while mid-to-high frequencies achieve higher D50 (more directional reflections, more absorption). ISO 3382-1 recommends computing D50 at individual octave bands and reporting the weighted average across 500 Hz to 4 kHz.
Relationship to STI
D50 and STI both predict speech intelligibility but use different measurement approaches. D50 is derived from the impulse response and captures the early-to-total energy ratio. STI measures the preservation of speech modulation patterns. In practice, rooms with D50 above 50% typically achieve STI above 0.50 ("Fair" to "Good"). However, STI additionally accounts for background noise, which D50 does not.
Try It Now
Measure D50 — free impulse response analysis in your browser